WARNING: All user stored data will be deleted. If the current settings and data are not expendable, create a backup before executing the Factory Reset. Reference: Owner's Manual, p.266-274.
1) Turn on the power.
2) While pressing the [VOICE] button, press and hold the [BANK D] button, then the  button. The SY99 will run the INITIAL TEST routine and display the Test Program Menu.
3) Release all held buttons and press the [COPY] button. The SY99 will execute Test 48 'Factory Settings', automatically exit the Test Mode, and then return to Play Mode.